1. Field of Invention
The invention relates to a sheet feeder that feeds cut sheets and an image forming apparatus including the sheet feeder, and more particularly, to a device that separates and feeds sheets, one by one, by abutting leading edges of the sheets in a sheet feed direction against an inclined separating surface formed by separating plates.
2. Description of Related Art
Conventional image forming apparatuses, such as laser-beam printers, color ink-jet printers, facsimile machines and copying machines, include a sheet feeder that feeds cut sheets, one by one, to an image forming unit provided therein. As disclosed in U.S. Pat. No. 6,158,733 and Japanese Laid-Open Patent Publication No. 2001-278507, the sheet feeder includes an inclined sheet holder, which holds a stack of sheets thereon, a separating plate, which is disposed below the sheet holder and has an inclined separating surface, and a sheet feed roller that is attached to an end of a rotatable arm having a predetermined length so as to face the sheet holder. The inclined separating surface extends such that the inclined separating surface and an upper surface of the sheet holder form an obtuse angle therebetween. Thus, the sheets stacked on the sheet holder are held by the inclined separating surface of the separating plate with the leading edges of the sheets contacting the inclined separating surface.
As the sheet feed roller rotates while pressing a topmost sheet in the stack on the sheet holder, the leading edge of the topmost sheet, which is being downwardly fed, abuts against the inclined separating surface. At that time, with a reaction force from the inclined separating surface, the topmost sheet is conveyed in a state where the lower part of the sheet is bent in a direction orthogonal to a direction that the rest of the sheets extend (the topmost sheet is convexedly warped such that the surface of the topmost sheet comes away from the stack of the sheets). Before long, the trailing edge of the sheet comes away from the inclined separating surface and thus only the topmost sheet is separated from the stack of the sheets. After that, the separated sheet is fed to the image forming unit by a conveying roller provided in a sheet feed path, to form an image thereon. Then, the sheet having the image is ejected from the image forming apparatus.
U.S. Pat. No. 6,158,733 discloses a sheet feeder that includes a feed tray, a support, which has an inclined holding surface provided below the feed tray, and a hopper plate. Between a lower end of the feed tray and the support, the hopper plate having a center of rotation on the side of the lower end of the feed tray, is inclinatorily provided. A compression spring is provided on the reverse side of the feed tray. A semi-cylindrical feed roller is fixedly provided facing an upper surface of the hopper plate. When a large number of sheets are stacked on the feed tray, the hopper plate retracts against the urging force from the compression spring, and thus, all of the stacked sheets are held by the support.
In addition, U.S. Pat. No. 6,158,733 discloses auxiliary holders that selectively protrude and retract with respect to an upper surface of the support (the inclined holding surface) to receive leading edges of the sheets. The upper surface of the support has a high frictional resistance. When rigid or strong sheets are fed from the feed tray, the auxiliary holders, having a low frictional resistance, protrude so that the sheets can be easily separated, one by one, from the stack of sheets. When soft or weak sheets are fed from the feed tray, the auxiliary holders retracts so that the leading edges of the sheets are held by the support having a high frictional resistance. Thus, the sheets are separated, one by one, from the stack of sheets.
However, in sheet feeders like the sheet feeder disclosed in U.S. Pat. No. 6,158,733, a distance between the sheet feed roller and the upper surface of the feed tray is maintained constant. Because of this structure, a maximum number of sheets that can be loaded on the feed tray is limited. In order to solve this problem, in recent sheet feeders, a sheet feed roller is rotatably supported by an arm, which can rotate about a center of rotation provided at a position upstream of the sheet feed roller in a sheet advance direction.
In the recent sheet feeders, a separating portion having a high coefficient of friction is provided to a portion of an inclined separating surface so as to surely separate sheets one by one even when the sheets are soft or weak. In this case, however, even when an angle (exterior angle) θ of less than 90 degrees formed between an extension of the surface of the feed tray and the inclined holding surface is constant, the leading edges of the sheets abut against the separating portion and thus lower parts of the sheets bend. Therefore, an angle between the sheets and the inclined separating surface becomes smaller than the angle θ, and the sheets are separated under this condition.
When rigid or strong sheets are fed from such the feed tray by the feed roller, the angle between the leading edges of the sheets and the inclined separating surface is not changed from the angle θ because lower parts of the sheets are hard to bend. Accordingly, the sheets are caught on the separating portion, and the sheet separation cannot be accomplished.